Thanks for the advice. I've spent the morning searching on Aliexpress searching, and there seems to be plenty of nylon spur gears available. Unfortunately I can't seem to find any replacements for the helical gear, and I'll need to confirm the sizes of the spur gears once I get back home. I've also sent an email to Gimson Robotics asking them what kind of gears they used themselves, and if they had replacements, though I reckon I'll need to wait for the new year before getting a reply.